:root {
        --navH: 90px;
    }

    /* <-- match your header height */

    html,
    body {
        margin: 0 !important;
        padding: 0 !important;
        height: 100% !important;
    }

    /* The wrapper takes the full viewport, but "pads down" by nav height */
    #blog-embed-wrap {
        width: 100%;
        height: 100vh;
        padding-top: var(--navH);
        /* <-- this creates the white space */
        box-sizing: border-box;
        overflow: hidden;
        background: #fff;
        /* <-- white space under nav */
    }

    #blog-embed-wrap iframe {
        width: 100%;
        height: 100%;
        border: 0;
        display: block;
    }